Tightly combined gps/bds carrier differential positioning method

ABSTRACT

A tightly combined GPS/BDS carrier differential positioning method is provided. The method comprises: using a GPS as a reference system to construct a GPS intra-system double-difference ionosphere-free combination model and a GPS/BDS inter-system double-difference ionosphere-free combination model; selecting a BDS reference satellite to re-parameterize an ambiguity of a GPS/BDS inter-system double-difference ionosphere-free combination and perform parameter decorrelation, estimating an ionosphere-free combination carrier differential inter-system bias in real time, and performing reference conversion on the ionosphere-free carrier inter-system bias to realize a continuous estimability of the ionosphere-free carrier differential inter-system bias in necessary; and finally, using ambiguity-fixed base carrier observations to form the ionosphere-free combination and performing tightly combined positioning on the inter-system double-difference ionosphere-free combination based on the estimated ionosphere-free carrier difference inter-system bias.

BACKGROUND Technical Field

The present invention relates to a multi-system combined navigation positioning technology, and more particularly, to a GPS/BDS tight combined carrier differential positioning method, which belongs to the field of GNSS (Global Navigation Satellite System) positioning and navigation technologies.

Description of Related Art

In relative positioning, two models are usually used when observations are combined by different satellite systems: one is that each system selects a loosely combined model of a reference satellite thereof, i.e., an intra-system difference model; and the other is that different systems select a tight combined model of a common reference satellite, i.e., an inter-system difference model. For a CDMA (Code Division Multiple Access) system, a satellite can eliminate carrier and pseudorange hardware delays at a receiver when performing intra-system difference. However, when performing inter-system difference, it is usually difficult to eliminate the hardware delay due to different signal modulation modes used by each system, and it is necessary to extract a difference inter-system bias as prior information for tight combined positioning.

Researches on tightly combined positioning mainly focus on the same frequencies of different systems at current, and are mainly applied to a single-frequency positioning model. In the process of combining multi-GNSS observations, different frequencies will be encountered more often, for example, GPS/BDS-2 dual systems do not have a shared frequency. Therefore, a differential positioning algorithm which only studies the same frequency between systems is not conducive to give full play to the advantages of multi-GNSS combined positioning.

The present research results show that the carrier difference inter-system bias between different systems with different frequencies presents time domain stability, which provides a technical basis for carrier difference tight combined positioning.

SUMMARY

In order to make up for the deficiency of the existing research and better exert the advantages of multi-GNSS tight combined positioning, the present invention provides a GPS/BDS tight combined carrier differential positioning method, which uses a GPS/BDS observation to construct an inter-system double-difference model, introduces a BDS reference satellite to perform parameter decorrelation, ensures a continuous estimability of a carrier difference inter-system bias through reference conversion, and finally, uses a fixed ambiguity to form an ionosphere-free combination and perform tight combined positioning based on the estimated carrier difference inter-system bias.

The following technical solutions are employed in the present invention to solve the technical problems above.

The present invention provides a GPS/BDS tight combined carrier differential positioning method, which comprises the following steps of:

step 1: selecting a GPS reference satellite to construct a GPS intra-system double-difference ionosphere-free combination model, a GPS/BDS inter-system double-difference ionosphere-free combination model and a GPS/BDS intra-system double-difference wide-lane ambiguity calculation model;

step 2: realizing decorrelation of an inter-system bias parameter with single-difference and double-difference ambiguities in ionosphere-free combinations;

step 3: performing reference conversion to realize a continuous estimability of an ionosphere-free combination difference inter-system bias;

step 4: separating a base carrier ambiguity by using an ionosphere-free combination and a fixed wide-lane ambiguity; and

step 5: forming the ionosphere-free combination by using base carrier observations to perform high-precision positioning.

As a further technical solution of the present invention, the step 1 specifically comprises:

step 11: constructing an inter-station single-difference ionosphere-free combination model:

$\begin{matrix} \begin{matrix} {{\Delta\varphi}_{{IF},G}^{s} = {{\Delta\rho}_{G}^{s} + {\Delta \; {dt}} + {\lambda_{{NL},G}\left( {{\Delta \; \delta_{{IF},G}} + {\Delta \; N_{{IF},G}^{s}}} \right)} + {\Delta \; T_{G}^{s}} + {\Delta ɛ}_{{IF},G}^{s}}} & \; \end{matrix} & (1) \\ \begin{matrix} {{\Delta \; P_{{IF},G}^{s}} = {{\Delta\rho}_{G}^{s} + {\Delta \; {dt}} + {\Delta \; d_{{IF},G}} + {\Delta \; T_{G}^{s}} + {\Delta \; e_{G}^{s}}}} & \; \end{matrix} & (2) \\ \begin{matrix} {{\Delta\varphi}_{{IF},C}^{q} = {{\Delta\rho}_{C}^{q} + {\Delta \; {dt}} + {\lambda_{{NL},C}\left( {{\Delta\delta}_{{IF},C} + {\Delta \; N_{{IF},C}^{q}}} \right)} + {\Delta \; T_{C}^{q}} + {\Delta ɛ}_{{IF},C}^{q}}} & \; \end{matrix} & (3) \\ \begin{matrix} {{\Delta \; P_{{IF},C}^{q}} = {{\Delta\rho}_{C}^{q} + {\Delta \; {dt}} + {\Delta \; d_{{IF},C}} + {\Delta \; T_{C}^{q}} + {\Delta \; e_{{IF},C}^{q}}}} & \; \end{matrix} & (4) \\ \begin{matrix} {{\Delta\varphi}_{{IF},G}^{s} = {\frac{f_{1,G}^{2}{\Delta\varphi}_{1,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}} - \frac{f_{2,G}^{2}{\Delta\varphi}_{2,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}}}} & {{\Delta \; N_{{IF},G}^{s}} = {\frac{f_{1,G}^{2}\Delta \; N_{1,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}} - \frac{f_{2,G}^{2}\Delta \; N_{2,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}}}} \end{matrix} & (5) \\ \begin{matrix} {{\Delta\varphi}_{{IF},C}^{q} = {\frac{f_{1,C}^{2}{\Delta\varphi}_{1,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}} - \frac{f_{2,G}^{2}{\Delta\varphi}_{2,C}^{q}}{f_{1,G}^{2} - f_{2,C}^{2}}}} & {{\Delta \; N_{{IF},C}^{q}} = {\frac{f_{1,C}^{2}\Delta \; N_{1,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}} - \frac{f_{2,C}^{2}\Delta \; N_{2,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}}}} \end{matrix} & (6) \\ \begin{matrix} {{\Delta \; P_{{IF},G}^{s}} = {\frac{f_{1,G}^{2}\Delta \; P_{1,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}} - \frac{f_{2,G}^{2}\Delta \; P_{2,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}}}} & {{\Delta \; P_{{IF},C}^{q}} = {\frac{f_{1,C}^{2}\Delta \; P_{1,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}} - \frac{f_{2,G}^{2}\Delta \; P_{2,C}^{q}}{f_{1,G}^{2} - f_{2,C}^{2}}}} \end{matrix} & (7) \end{matrix}$

wherein equations (1) and (2) respectively represent a carrier observation equation and a pseudorange observation equation of a GPS inter-station single-difference ionosphere-free combination, equations (3) and (4) respectively represent a subcarrier observation equation and a pseudorange observation equation of a BDS inter-station single-difference ionosphere-free combination, equation (5) represents a GPS inter-station single-difference ionosphere-free carrier observation and a GPS inter-station single-difference ionosphere-free ambiguity, equation (6) represents a BDS inter-station single-difference ionosphere-free carrier observation value and a BDS inter-station single-difference ionosphere-free ambiguity, and equation (7) represents a GPS inter-station single-difference pseudorange ionosphere-free combination and a BDS inter-station single-difference pseudorange ionosphere-free combination;

wherein, s=1_(G),2_(G), . . . , m_(G), m_(G) represents a number of GPS satellites, Δϕ_(IF,G) ^(s) represents a carrier observation of an inter-station single-difference ionosphere-free combination of GPS satellite s, Δρ_(G) ^(s) represents a single-difference distance between a station and the GPS satellite, Δρ_(G) ^(s) represents an inter-station single-difference receiver clock bias, λ_(NL,G) represents a GPS narrow-lane wavelength, Δδ_(IF,G) represents a carrier hardware delay of an inter-station single-difference ionosphere-free combination of a GPS satellite receiver, ΔN_(IF,G) ^(s) represents an ambiguity of the inter-station single-difference ionosphere-free combination of the GPS satellite s, ΔT_(G) ^(s) represents an inter-station single-difference troposphere delay of the GPS satellite, Δε_(IF,G) ^(s) represents the measurement noise of the inter-station single-difference ionosphere-free combination of the GPS satellite, ΔP_(IF,G) ^(s) represents a pseudorange observation of the inter-station single-difference ionosphere-free combination of the GPS satellite s, Δd_(IF,G) represents a pseudorange hardware delay of the inter-station single-difference ionosphere-free combination of the GPS satellite receiver end, and Δe_(IF,G) ^(s) represents a pseudorange measured noise of the inter-station single-difference ionosphere-free combination of the GPS satellite s; q=1_(C),2_(C), . . . , n_(C), n_(C) represents a number of BDS satellites, Δϕ_(IF,C) ^(q) represents a carrier observation value of an inter-station single-difference ionosphere-free combination of BDS satellite q, Δρ_(C) ^(q) represents an inter-station single-difference station satellite distance of BDS satellite q, λ_(NL,C) represents a BDS narrow-lane wavelength, Δδ_(IF,C) represents a carrier hardware delay of an inter-station single-difference ionosphere-free combination of the BDS receiver, ΔN_(IF,C) ^(q) represents the ambiguity of the inter-station single-difference ionosphere-free combination of BDS satellite q, ΔT_(C) ^(q) represents an inter-station single-difference troposphere delay of BDS satellite q, Δε_(IF,C) ^(q) represents a measurement noise of the inter-station single-difference ionosphere-free combination of BDS satellite q, ΔP_(IF,C) ^(q) represents a pseudorange observation of the inter-station single-difference ionosphere-free combination of BDS satellite q, Δd_(IF,C) represents a pseudorange hardware delay of the inter-station single-difference ionosphere-free combination of the BDS satellite receiver, and Δe_(IF,C) ^(q) represents a pseudorange measurement noise of the inter-station single-difference ionosphere-free combination of BDS satellite q; Δϕ_(1,G) ^(s) represents an inter-station single-difference carrier observation on L frequency of GPS satellite s, Δϕ_(2,G) ^(s) represents an inter-station single-difference carrier observation on L2 frequency of GPS satellite s, ΔN_(1,G) ^(s) represents an inter-station single-difference ambiguity on L1 frequency of GPS satellite s, ΔN_(2,G) ^(s) represents an inter-station single-difference ambiguity on L2 frequency of GPS satellite s, ΔP_(1,G) ^(s), represents an inter-station single-difference pseudorange observation on L1 frequency of GPS satellites s, ΔP_(2,G) ^(s) represents an inter-station single-difference pseudorange observation on L2 frequency of GPS satellites s, f_(1,G) represents a GPS L frequency, and f_(2,G) represents a GPS L2 frequency; and Δ_(1,C) ^(q) represents an inter-station single-difference carrier observation on B1 frequency of BDS satellite q, Δϕ_(2,C) ^(q) represents an inter-station single-difference carrier observation on B2 frequency of BDS satellite q, ΔN_(1,C) ^(q) represents the inter-station single-difference ambiguity on B1 frequency of the BDS satellite q, ΔN_(2,C) ^(q) represents the inter-station single-difference ambiguity on B2 frequency of the BDS satellite q, ΔP_(1,C) ^(q) represents the inter-station single-difference pseudorange observation on B1 frequency of BDS satellite q, ΔP_(2,C) ^(q) represents the inter-station single-difference pseudorange observation on B2 frequency of the BDS satellite q, f_(1,C) represents a B1 frequency of BDS, and f_(2,C) represents a B2 frequency of BDS;

step 12: selecting a GPS reference satellite to construct the GPS intra-system double-difference ionosphere-free combination model and the GPS/BDS inter-system double-difference ionosphere-free combination model according to the inter-station single-difference ionosphere-free combination model constructed in the step 11:

when a GPS satellite 1_(G) is used as a reference satellite, equations (8) and (9) representing GPS intra-system double-difference ionosphere-free combination models, and equations (10) and (11) representing GPS/BDS inter-system double-difference ionosphere-free combination models:

$\begin{matrix} {{\nabla{\Delta\varphi}_{{IF},G}^{1_{G},s}} = {{\nabla{\Delta\rho}_{G}^{1_{G},s}} + {\lambda_{{NL},G}\Delta \; N_{{IF},G}^{1_{G},s}} + {{\nabla\Delta}\; T_{G}^{1_{G},s}} + {\nabla{\Delta ɛ}_{{IF},G}^{1_{G},s}}}} & (8) \\ {{{\nabla\Delta}\; P_{{IF},G}^{1_{G},s}} = {{\nabla{\Delta\rho}_{G}^{1_{G},s}} + {{\nabla\Delta}\; T_{G}^{1_{G},s}} + {{\nabla\Delta}\; e_{{IF},G}^{1_{G},s}}}} & (9) \\ {{\nabla{\Delta\varphi}_{{IF},{GC}}^{1_{G},q}} = {{{\Delta\varphi}_{{IF},C}^{q} - {\Delta\varphi}_{{IF},G}^{1_{G}}} = {{\nabla{\Delta\rho}_{GC}^{1_{G},q}} + {\lambda_{{NL},C}{\nabla\Delta}\; N_{{IF},{GC}}^{1_{G},q}} + {\left( {\lambda_{{NL},C} - \lambda_{{NL},G}} \right)\Delta \; N_{{IF},G}^{1_{G}}} + {\lambda_{{NL},C}{\nabla{\Delta\delta}_{{IF},{GC}}}} + {{\nabla\Delta}\; T_{GC}^{1_{G},q}} + {\nabla{\Delta ɛ}_{{IF},{GC}}^{1_{G},q}}}}} & (10) \\ {{{\nabla\Delta}\; P_{{IF},{GC}}^{1_{G},q}} = {{{\Delta \; P_{{IF},C}^{q}} - {\Delta \; P_{{IF},G}^{1_{G}}}} = {{\nabla{\Delta\rho}_{GC}^{1_{G},q}} + {{\nabla\Delta}\; d_{{IF},{GC}}} + {{\nabla\Delta}\; T_{GC}^{1_{G},q}} + {{\nabla\Delta}\; e_{{IF},{GC}}^{1_{G},q}}}}} & (11) \end{matrix}$

wherein, ∇Δϕ_(IF,G) ¹ ^(G) ^(,s) represents a carrier observation of the GPS intra-system double-difference ionosphere-free combination, ∇Δρ_(G) ¹ ^(G) ^(,s) represents a GPS intra-system double-difference distance between stations and satellites, ΔN_(IF,G) ¹ ^(G) ^(,s) represents a double-difference ambiguity of the GPS intra-system ionosphere-free combination, ∇ΔT_(G) ¹ ^(G) ^(,s) represents a GPS intra-system double-difference troposphere delay, ∇Δε_(IF,G) ¹ ^(G) ^(,s) represents a carrier observation of the GPS intra-system double-difference ionosphere-free combination, ∇ΔP_(IF,G) ¹ ^(G) ^(,s) represents a pseudorange observation of the GPS intra-system double-difference ionosphere-free combination, and ∇Δe_(IF,G) ¹ ^(G) ^(,s) represents a carrier measurement noise of the GPS intra-system double-difference ionosphere-free combination; and ∇Δϕ_(IF,GC) ¹ ^(G) ^(,q) represents a carrier observation of the GPS/BDS inter-system double-difference ionosphere-free combination, ∇Δρ_(GC) ¹ ^(G) ^(,q) represents a GPS/BDS inter-system double-difference distance between satellites and stations, ∇ΔN_(IF,GC) ¹ ^(G) ^(,q) represents an ambiguity of the GPS/BDS inter-system double-difference ionosphere-free combination, ΔN_(IF,G) ¹ ^(G) represents an ambiguity of the inter-station single-difference ionosphere-free combination of the GPS reference satellite,

${\nabla{\Delta\delta}_{{IF},{GC}}} = {{\Delta\delta}_{{IF},C} - {\frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}{\Delta\delta}_{{IF},G}}}$

represents a carrier difference inter-system bias of the GPS/BDS ionosphere-free combination, ∇Δ_(GC) ¹ ^(G) ^(,q) represents a GPS/BDS inter-system double-difference troposphere delay, ∇Δε_(IF,GC) ¹ ^(G) ^(,q) represents a carrier observation of the GPS/BDS inter-system double-difference ionosphere-free combination, ∇ΔP_(IF,GC) ¹ ^(G) ^(,q) represents a pseudorange observation of the GPS/BDS inter-system double-difference ionosphere-free combination, ∇Δd_(IF,GC)=Δd_(IF,C)−Δd_(IF,G) represents a GPS/BDS pseudorange differential inter-system bias of the ionosphere-free combination, and ∇Δe_(IF,GC) ¹ ^(G) ^(,q) represents a pseudorange observation of the GPS/BDS inter-system double-difference ionosphere-free combination; and

step 13: selecting a BDS reference satellite to construct intra-system double-difference wide-lane ambiguity calculation models of GPS and BDS:

when a BDS satellite 1_(C) is used as a BDS reference satellite, then the respective intra-system double-difference wide-lane ambiguity calculation models of the GPS and the BDS being:

$\begin{matrix} {{{\nabla\Delta}\; N_{{WL},G}^{1_{G},s}} = {{\nabla{\Delta\phi}_{{WL},G}^{1_{G},s}} - \frac{{f_{1,G}{\nabla\Delta}\; P_{1,G}^{1_{G},s}} + {f_{2,G}{\nabla\Delta}\; P_{2,G}^{1_{G},s}}}{\lambda_{{WL},G}\left( {f_{1,G} + f_{2,G}} \right)}}} & (12) \\ {{{\nabla\Delta}\; N_{{WL},C}^{1_{C},q}} = {{\nabla{\Delta\phi}_{{WL},C}^{1_{C},q}} - \frac{{f_{1,C}{\nabla\Delta}\; P_{1,C}^{1_{C},q}} + {f_{2,C}{\nabla\Delta}\; P_{2,C}^{1_{C},q}}}{\lambda_{{WL},C}\left( {f_{1,C} + f_{2,C}} \right)}}} & (13) \end{matrix}$

wherein, ∇ΔN_(WL,G) ¹ ^(G) ^(,s) represents a GPS double-difference wide-lane ambiguity, ∇Δφ_(WL,G) ¹ ^(G) ^(,s) represents a GPS double-difference wide-lane carrier observation, ∇ΔP_(1,G) ¹ ^(G) ^(,s) represents a GPS L1 double-difference pseudorange observation, ∇ΔP_(2,G) ¹ ^(G) ^(,s) represents a GPS L2 double-difference pseudorange observation, and λ_(WL,G) represents a GPS wide-lane wavelength; and ∇ΔN_(WL,C) ¹ ^(C) ^(,q) represents a BDS double-difference wide-lane ambiguity, ∇Δϕ_(WL,C) ¹ ^(C) ^(,q) represents a BDS double-difference wide-lane carrier observation, ∇ΔP_(C) ¹ ^(C) ^(,q) represents a BDS B1 double-difference pseudorange observation, ∇ΔP_(2,C) ¹ ^(C) ^(,q) represents a BDS B2 double-difference pseudorange observation, and λ_(WL,C) represents a BDS wide-lane wavelength; and

performing multi-epoch smooth rounding on equations (12) and (13) to obtain a double-difference wide-lane whole-cycle ambiguity:

$\begin{matrix} \begin{matrix} {{{\nabla\overset{\Cap}{\Delta}}\; {\overset{\_}{N}}_{{WL},G}^{1_{G},s}} = {{round}\mspace{14mu} \left( {\frac{1}{k}{\sum\limits_{i = 1}^{k}{{\nabla\overset{\Cap}{\Delta \;}}N_{{WL},G}^{1_{G},s}}}} \right)}} \\ {{{\nabla\overset{\Cap}{\Delta}}\; {\overset{\_}{N}}_{{WL},C}^{1_{C},q}} = {{round}\mspace{14mu} \left( {\frac{1}{k}{\sum\limits_{i = 1}^{k}{{\nabla\overset{\Cap}{\Delta \;}}N_{{WL},C}^{1_{C},q}}}} \right)}} \\ {{k \in N^{+}}\mspace{315mu}} \end{matrix} & (14) \end{matrix}$

wherein, ∇{circumflex over (Δ)}N _(WL,G) ¹ ^(G) ^(,s) and ∇{circumflex over (Δ)}N _(WL,C) ¹ ^(C) ^(,q) are respectively the double-difference wide-lane whole-cycle ambiguities of the GPS and the BDS obtained by multi-epoch smooth rounding, round represents a rounding operator, and k represents an epoch number.

As a further technical solution of the present invention, the step 2 specifically comprises:

step 21: reparameterizing the ambiguity of the GPS/BDS inter-system double-difference ionosphere-free combination according to the BDS reference satellite selected in the step 13:

according to the step 12, the ambiguity of the GPS/BDS inter-system double-difference ionosphere-free combination being represented as:

∇ΔN _(IF,G) ¹ ^(G) ^(,q)=(ΔN _(IF,C) ^(q) −ΔN _(IF,C) ¹ ^(C) )+(ΔN _(IF,C) ¹ ^(C) −ΔN _(IF,G) ¹ ^(G) )=∇ΔN _(IF,C) ¹ ^(C) ^(,q) +∇ΔN _(IF,GC) ¹ ^(G) ¹ ^(C)   (15)

wherein, ΔN_(IF,C) ¹ ^(c) represents an ambiguity of an inter-station single-difference ionosphere-free combination of a BDS reference satellite, ∇ΔN_(IF,C) ¹ ^(C) ^(,q) represents an ambiguity of a BDS intra-system double-difference ionosphere-free combination, and ∇ΔN_(IF,GC) ¹ ^(G) ¹ ^(C) represents ambiguities of the GPS/BDS inter-system double-difference ionosphere-free combinations of the BDS reference satellite and the GPS reference satellite;

according to equation (15), equation (10) being represented as:

∇Δϕ_(IF,GC) ¹ ^(G) ^(,q)=∇Δρ_(GC) ¹ ^(G) ^(,q)+λ_(NL,C) ∇ΔN _(IF,C) ¹ ^(C) ^(,q)+λ_(NL,C) ∇ΔN _(IF,GC) ¹ ^(G) ¹ ^(C) +(λ_(IF,C)−λ_(IF,G))ΔN _(IF,G) ¹ ^(G) +λ_(NL,C)∇Δδ_(IF,GC) +∇ΔT _(GC) ¹ ^(G) ^(,q)+∇Δε_(IF,GC) ¹ ^(G) ^(,q)  (16)

wherein, in equation (10), ∇ΔN_(IF,GC) ¹ ^(G) ¹ ^(C) , ΔN_(IF,G) ¹ ^(G) and ∇Δδ_(IF,GC) are parameters shared by all BDS satellites and are linearly correlated; and

step 22: combining the shared parameters and reparametrizing the ionosphere-free combination carrier difference inter-system bias to realize parameter decorrelation:

according to equation (16), an observation equation of the GPS/BDS inter-system double-difference ionosphere-free combination after the shared parameters are combined being represented as:

∇Δϕ_(IFGC) ¹ ^(G) ^(,q)=∇Δρ_(GC) ¹ ^(G) ^(,q)+λ_(NL,C) ∇ΔN _(NL,C) ¹ ^(C) ^(,q)+λ_(NL,C)∇Δδ _(IF,GC) +∇ΔT _(GC) ¹ ^(G) ^(,q)+∇Δε_(IF,GC) ¹ ^(G) ^(,q)  (17)

wherein, ∇Δδ _(IF,GC) represents an ionosphere-free combination carrier difference inter-system bias after reparameterization, and

${{\nabla\Delta}{\overset{\_}{\delta}}_{{IF},{GC}}} = {{{\nabla\Delta}\; N_{{IF},{GC}}^{1_{G}1_{C}}} + {\nabla{\Delta\delta}_{{IF},{GC}}} + {\left( {1 - \frac{\lambda_{{NL},C}}{\lambda_{{NL},G}}} \right)\Delta \; {N_{{IF},G}^{1_{G}}.}}}$

As a further technical solution of the present invention, the step 3 specifically comprises:

step 31: performing GPS reference conversion:

assuming that the GPS reference satellite is converted from 1_(G) into i_(G) at a t^(th) epoch, a corresponding ionosphere-free combination carrier difference inter-system bias ∇Δδ _(IF,GC)(t) of the t^(th) epoch being:

$\begin{matrix} {{{\nabla\Delta}{{\overset{\_}{\delta}}_{{IF},{GC}}(t)}} = {{{{\nabla\Delta}{{\overset{\_}{\delta}}_{{IF},{GC}}\left( {t - 1} \right)}} - {\frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}{\nabla\Delta}\; N_{{IF},G}^{1_{G},i_{G}}}} = {{\nabla{\Delta\delta}_{{IF},{GC}}} + {{\nabla\Delta}\; N_{{IF},{GC}}^{i_{G},1_{C}}} + {\left( {1 - \frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}} \right)\Delta \; N_{{IF},G}^{i_{G}}}}}} & (18) \end{matrix}$

wherein, ∇Δδ _(IF,GC)(t−1) is an ionosphere-free combination carrier difference inter-system bias of a (t−1)^(th) epoch; and

step 32: performing BDS reference conversion:

assuming that the BDS reference satellite is converted from 1_(C) into i_(C) at a j^(th) epoch, while the GPS reference satellite at the moment being i_(G) in the step 31, then a corresponding ionosphere-free combination carrier difference inter-system bias ∇Δδ _(IF,GC) (j) of the j^(th) epoch being:

$\begin{matrix} {{{\nabla\Delta}{{\overset{\_}{\delta}}_{{IF},{GC}}(j)}} = {{{{\nabla\Delta}{{\overset{\_}{\delta}}_{{IF},{GC}}\left( {j - 1} \right)}} + {{\nabla\Delta}\; N_{{IF},{GC}}^{1_{G},i_{C}}}} = {{\nabla{\Delta\delta}_{{IF},{GC}}} + {{\nabla\Delta}\; N_{{IF},{GC}}^{i_{G},i_{C}}} + {\left( {1 - \frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}} \right)\Delta \; N_{{IF},G}^{i_{G}}}}}} & (19) \end{matrix}$

wherein, ∇Δδ _(IF,GC)(j−1) is an ionosphere-free combination difference carrier inter-system bias of a (j−1)^(th) epoch;

so far, the continuous estimability of the ionosphere-free combination carrier difference inter-system bias is realized.

As a further technical solution of the present invention, the step 4 specifically comprises:

step 41: separating the ambiguity of the GPS L1 and the ambiguity of BDS B1 according to the wide-lane ambiguity obtained in the step 13 by combining the ionosphere-free combination with a wide-lane combination:

$\begin{matrix} {{{\nabla\Delta}\; N_{1,G}^{1_{G},s}} = {{{\nabla\Delta}\; N_{{IF},G}^{1_{G},s}} - {\frac{f_{2,G}}{f_{1,G} - f_{2,G}}{\nabla\Delta}\; {\overset{\_}{N}}_{{WL},G}^{1_{G},s}}}} & (20) \\ {{{\nabla\Delta}\; N_{1,C}^{1_{C},q}} = {{{\nabla\Delta}\; N_{{IF},C}^{1_{C},q}} - {\frac{f_{2,C}}{f_{1,C} - f_{2,C}}{\nabla\Delta}\; {\overset{\_}{N}}_{{WL},C}^{1_{C},q}}}} & (21) \end{matrix}$

wherein, ∇ΔN_(1,G) ¹ ^(G) ^(,s) is a separated ambiguity float solution of the GPS L1, ∇ΔN_(1,C) ¹ ^(C) ^(,q) is a separated ambiguity float solution of the BDS B1, ∇ΔN _(1,G) ¹ ^(G) ^(,s) is an integer-ambiguity solution of the GPS L1, and ∇ΔN _(1,C) ¹ ^(C) ^(,q) is an integer-ambiguity solution of the BDS B1; and

step 42: according to the wide-lane ambiguity obtained in the step 13 and the ambiguity of the GPS L1 and the ambiguity of the BDS B1 obtained in the step 41, calculating an integer-ambiguity solution of the GPS L2 and an integer-ambiguity solution of the BDS L2:

∇Δ N _(2,G) ¹ ^(G) ^(,s) =∇ΔN _(1,G) ¹ ^(G) ^(,s) −∇ΔN _(WL,G) ¹ ^(G) ^(,s) ,∇ΔN _(2,C) ¹ ^(C) ^(,q) =∇ΔN _(1,C) ¹ ^(C) ^(,q) −∇ΔN _(WL,C) ¹ ^(C) ^(,q)  (22)

wherein, ∇ΔN _(2,G) ¹ ^(G) ^(,s) and ∇ΔN _(1,C) ¹ ^(C) ^(,q) are respectively the integer-ambiguity solutions of the GPS L2 and the BDS B2.

As a further technical solution of the present invention, the integer-ambiguity solutions ∇ΔN _(1,G) ¹ ^(G) ^(,s) and ∇ΔN _(1,C) ¹ ^(C) ^(,q) of the GPS L1 and the BDS B1 are searched by a least-squares ambiguity decorrelation adjustment (LAMBDA) method.

As a further technical solution of the present invention, the step 5 specifically comprises: forming the double-difference ionosphere-free combination according to the step 21 based on the integer-ambiguity solutions and the carrier observations obtained in the step 41 and the step 42, and substituting the formed ionosphere-free combination and the ionosphere-free combination carrier difference inter-system bias obtained in the step 2 into the equations (5) and (7) for positioning.

Compared with the prior art, the present invention employing the above technical solutions has the following technical effects:

(1) the present invention uses GNSS inter-system observations of different frequencies to perform the carrier difference tight combined positioning, thus overcoming the defect that the inter-system observations must have the same frequency in the present researches; and

(2) the present invention can reduce parameters to be estimated, which is conductive to enhance the stability of an observation model in an shielded environment, and improves the positioning accuracy and reliability.

To make the aforementioned more comprehensible, several embodiments accompanied with drawings are described in detail as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure.

FIG. 1 is a seven-day positioning bias diagram of a loose combination in an N direction.

FIG. 2 is a seven-day positioning bias diagram of a tight combination in the N direction.

FIG. 3 is a seven-day positioning bias diagram of the loose combination in an E direction.

FIG. 4 is a seven-day positioning bias diagram of the tight combination in the E direction.

FIG. 5 is a seven-day positioning bias diagram of the loose combination in a U direction.

FIG. 6 is a seven-day positioning bias diagram of the tight combination in the U direction.

FIG. 7 is a flow chart of a method according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

The present invention will be further described with reference to the drawings and the specific embodiments. It should be understood that these embodiments are only used for illustrating the present invention and are not intended to limit the scope of the present invention, and modifications of various equivalent forms made by those skilled in the art on the present invention after reading the present invention, shall all fall within the scope defined by the appended claims of the present application.

The present invention provides a GPS/BDS tightly combined carrier differential positioning method, as shown in FIG. 7, comprising the following steps of:

step 1: selecting a GPS reference satellite to construct a GPS intra-system double-difference ionosphere-free combination model, a GPS/BDS inter-system double-difference ionosphere-free combination model and a GPS/BDS intra-system double-difference wide-lane ambiguity calculation model;

step 2: realizing decorrelation of an inter-system bias parameter with single-difference and double-difference ambiguities in ionosphere-free combinations;

step 3: performing reference conversion to realize a continuous estimability of an ionosphere-free combination difference inter-system bias;

step 4: separating a base carrier ambiguity by using an ionosphere-free combination and a fixed wide-lane ambiguity; and

step 5: forming the ionosphere-free combination by using base carrier observations to perform high-precision positioning.

The constructed GPS intra-system double-difference model and the GPS/BDS inter-system double-difference model in the step 1 comprises the following steps.

In step 11, an inter-station single-difference ionosphere-free combination model is constructed:

assuming that a total of m GPS satellites and n BDS satellites are observed, an observation model of an inter-station single-difference ionosphere-free combination may be represented as:

$\begin{matrix} \begin{matrix} {{\Delta\varphi}_{{IF},G}^{s} = {{\Delta\rho}_{G}^{s} + {\Delta \; {dt}} + {\lambda_{{NL},G}\left( {{\Delta \; \delta_{{IF},G}} + {\Delta \; N_{{IF},G}^{s}}} \right)} + {\Delta \; T_{G}^{s}} + {\Delta ɛ}_{{IF},G}^{s}}} & \; \end{matrix} & (1) \\ \begin{matrix} {{\Delta \; P_{{IF},G}^{s}} = {{\Delta\rho}_{G}^{s} + {\Delta \; {dt}} + {\Delta \; d_{{IF},G}} + {\Delta \; T_{G}^{s}} + {\Delta \; e_{G}^{s}}}} & \; \end{matrix} & (2) \\ \begin{matrix} {{\Delta\varphi}_{{IF},C}^{q} = {{\Delta\rho}_{C}^{q} + {\Delta \; {dt}} + {\lambda_{{NL},C}\left( {{\Delta\delta}_{{IF},C} + {\Delta \; N_{{IF},C}^{q}}} \right)} + {\Delta \; T_{C}^{q}} + {\Delta ɛ}_{{IF},C}^{q}}} & \; \end{matrix} & (3) \\ \begin{matrix} {{\Delta \; P_{{IF},C}^{q}} = {{\Delta\rho}_{C}^{q} + {\Delta \; {dt}} + {\Delta \; d_{{IF},C}} + {\Delta \; T_{C}^{q}} + {\Delta \; e_{{IF},C}^{q}}}} & \; \end{matrix} & (4) \\ \begin{matrix} {{\Delta\varphi}_{{IF},G}^{s} = {\frac{f_{1,G}^{2}{\Delta\varphi}_{1,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}} - \frac{f_{2,G}^{2}{\Delta\varphi}_{2,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}}}} & {{\Delta \; N_{{IF},G}^{s}} = {\frac{f_{1,G}^{2}\Delta \; N_{1,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}} - \frac{f_{2,G}^{2}\Delta \; N_{2,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}}}} \end{matrix} & (5) \\ \begin{matrix} {{\Delta\varphi}_{{IF},C}^{q} = {\frac{f_{1,C}^{2}{\Delta\varphi}_{1,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}} - \frac{f_{2,G}^{2}{\Delta\varphi}_{2,C}^{q}}{f_{1,G}^{2} - f_{2,C}^{2}}}} & {{\Delta \; N_{{IF},C}^{q}} = {\frac{f_{1,C}^{2}\Delta \; N_{1,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}} - \frac{f_{2,C}^{2}\Delta \; N_{2,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}}}} \end{matrix} & (6) \\ \begin{matrix} {{\Delta \; P_{{IF},G}^{s}} = {\frac{f_{1,G}^{2}\Delta \; P_{1,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}} - \frac{f_{2,G}^{2}\Delta \; P_{2,G}^{2}}{f_{1,G}^{2} - f_{2,G}^{2}}}} & {{\Delta \; P_{{IF},C}^{q}} = {\frac{f_{1,C}^{2}\Delta \; P_{1,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}} - \frac{f_{2,G}^{2}\Delta \; P_{2,C}^{q}}{f_{1,G}^{2} - f_{2,C}^{2}}}} \end{matrix} & (7) \end{matrix}$

wherein equations (1) and (2) respectively represent a carrier observation equation and a pseudorange observation equation of a GPS inter-station single-difference ionosphere-free combination, equations (3) and (4) respectively represent a subcarrier observation equation and a pseudorange observation equation of a BDS inter-station single-difference ionosphere-free combination, equation (5) represents a GPS inter-station single-difference ionosphere-free carrier observation and a GPS inter-station single-difference ionosphere-free ambiguity, equation (6) represents a BDS inter-station single-difference ionosphere-free carrier observation value and a BDS inter-station single-difference ionosphere-free ambiguity, and equation (7) represents a GPS inter-station single-difference pseudorange ionosphere-free combination and a BDS inter-station single-difference pseudorange ionosphere-free combination. The ionosphere-free combination forms shown in the equations (5), (6) and (7) are also applicable to a non-difference form and a double-difference form.

Δϕ_(IF,G) ^(s) (a superscript s=1_(G),2_(G), . . . , m_(G), m_(G) represents a GPS satellite) represents a carrier observation (m) of an inter-station single-difference ionosphere-free combination of a GPS satellite, Δρ_(G) ^(s) represents a single-difference distance between a station and the GPS satellite, Δdt represents an inter-station single-difference receiver clock bias, λ_(NL,G) represents a GPS narrow-lane wavelength, Δδ_(IF,G) represents a carrier hardware delay of an inter-station single-difference ionosphere-free combination of a GPS satellite receiver, ΔN_(IF,G) ^(s) represents an ambiguity of the inter-station single-difference ionosphere-free combination of the GPS satellite s, ΔT_(G) ^(s) represents an inter-station single-difference troposphere delay of the GPS satellite, Δε_(IF,G) ^(s) represents the measurement noise of the inter-station single-difference ionosphere-free combination of the GPS satellite, ΔP_(IF,G) ^(s) represents a pseudorange observation of the inter-station single-difference ionosphere-free combination of the GPS satellite s, Δd_(IF,G) represents a pseudorange hardware delay of the inter-station single-difference ionosphere-free combination of the GPS satellite receiver end, and Δe_(IF,G) ^(s) represents a pseudorange measured noise of the inter-station single-difference ionosphere-free combination of the GPS satellite s; and Δϕ_(IF,C) ^(q) (a superscript q=1_(C), 2_(C), . . . ,n_(C), n_(C) represents a BDS satellite) represents a carrier observation (m) of an inter-station single-difference ionosphere-free combination of a BDS satellite, Δρ_(C) ^(q) represents an inter-station single-difference station satellite distance of BDS satellite q, λ_(NL,C) represents a BDS narrow-lane wavelength, Δδ_(IF,C) represents a carrier hardware delay of an inter-station single-difference ionosphere-free combination of the BDS receiver, ΔN_(IF,C) ^(q) represents the ambiguity of the inter-station single-difference ionosphere-free combination of BDS satellite q, ΔT_(C) ^(q) represents an inter-station single-difference troposphere delay of BDS satellite q, Δε_(IF,C) ^(q) represents a measurement noise of the inter-station single-difference ionosphere-free combination of BDS satellite q, ΔP_(IF,C) ^(q) represents a pseudorange observation of the inter-station single-difference ionosphere-free combination of BDS satellite q, Δd_(IF,C) represents a pseudorange hardware delay of the inter-station single-difference ionosphere-free combination of the BDS satellite receiver, and Δe_(IF,C) ^(q) represents a pseudorange measurement noise of the inter-station single-difference ionosphere-free combination of BDS satellite q; Δϕ_(1,G) ^(s) represents an inter-station single-difference carrier observation on L frequency of GPS satellite s, Δ_(2,G) ^(s) represents an inter-station single-difference carrier observation on L2 frequency of GPS satellite s, ΔN_(1,G) ^(s) represents an inter-station single-difference ambiguity on L1 frequency of GPS satellite s, ΔN_(2,G) ^(s) represents an inter-station single-difference ambiguity on L2 frequency of GPS satellite s, ΔP_(1,G) ^(s) represents an inter-station single-difference pseudorange observation on L1 frequency of GPS satellites s, ΔP_(2,G) ^(s) represents an inter-station single-difference pseudorange observation on L2 frequency of GPS satellites s, f_(1,G) represents a GPS L frequency, and f_(2,G) represents a GPS L2 frequency; and Δϕ_(1,C) ^(s) represents an inter-station single-difference carrier observation on B1 frequency of BDS satellite q, Δϕ_(2,C) ^(q) represents an inter-station single-difference carrier observation on B2 frequency of BDS satellite q, ΔN_(1,C) ^(q) represents the inter-station single-difference ambiguity on B1 frequency of the BDS satellite q, ΔN_(2,C) ^(q) represents the inter-station single-difference ambiguity on B2 frequency of the BDS satellite q, ΔP_(1,C) ^(q) represents the inter-station single-difference pseudorange observation on B1 frequency of BDS satellite q, ΔP_(2,C) ^(q) represents the inter-station single-difference pseudorange observation on B2 frequency of the BDS satellite q, f_(1,C) represents a B1 frequency of BDS, and f_(2,C) represents a B2 frequency of BDS;

In step 12, a GPS reference satellite is selected to construct the GPS intra-system double-difference ionosphere-free combination model and the GPS/BDS inter-system double-difference ionosphere-free combination model according to the inter-station single-difference ionosphere-free combination model constructed in the step 11:

assuming that a GPS satellite 1_(G) is used as a reference satellite, the model constructed may be represented as:

$\begin{matrix} {{\nabla{\Delta\varphi}_{{IF},G}^{1_{G},s}} = {{\nabla{\Delta\rho}_{G}^{1_{G},s}} + {\lambda_{{NL},G}\Delta \; N_{{IF},G}^{1_{G},s}} + {{\nabla\Delta}\; T_{G}^{1_{G},s}} + {\nabla{\Delta ɛ}_{{IF},G}^{1_{G},s}}}} & (8) \\ {{{\nabla\Delta}\; P_{G}^{1_{G},s}} = {{\nabla{\Delta\rho}_{G}^{1_{G},s}} + {{\nabla\Delta}\; T_{G}^{1_{G},s}} + {{\nabla\Delta}\; e_{{IF},G}^{1_{G},s}}}} & (9) \\ {{\nabla{\Delta\varphi}_{{IF},{GC}}^{1_{G},q}} = {{{\Delta\varphi}_{{IF},C}^{q} - {\Delta\varphi}_{{IF},G}^{1_{G}}} = {{\nabla{\Delta\rho}_{GC}^{1_{G},q}} + {\lambda_{{NL},C}{\nabla\Delta}\; N_{{IF},{GC}}^{1_{G},q}} + {\left( {\lambda_{{NL},C} - \lambda_{{NL},G}} \right)\Delta \; N_{{IF},G}^{1_{G}}} + {\lambda_{{NL},C}{\nabla{\Delta\delta}_{{IF},{GC}}}} + {{\nabla\Delta}\; T_{GC}^{1_{G},q}} + {\nabla{\Delta ɛ}_{{IF},{GC}}^{1_{G},q}}}}} & (10) \\ {{{\nabla\Delta}\; P_{{IF},{GC}}^{1_{G},q}} = {{{\Delta \; P_{{IF},C}^{q}} - {\Delta \; P_{{IF},G}^{1_{G}}}} = {{\nabla{\Delta\rho}_{GC}^{1_{G},q}} + {{\nabla\Delta}\; d_{{IF},{GC}}} + {{\nabla\Delta}\; T_{GC}^{1_{G},q}} + {{\nabla\Delta}\; e_{{IF},{GC}}^{1_{G},q}}}}} & (11) \end{matrix}$

wherein, equations (8) and (9) represent GPS intra-system double-difference ionosphere-free combination models, and equations (10) and (11) represent GPS/BDS inter-system double-difference ionosphere-free combination models.

${\nabla{\Delta\delta}_{{IF},{GC}}} = {{\Delta\delta}_{{IF},C} - {\frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}{\Delta\delta}_{{IF},G}}}$

represents a carrier differential inter-system bias of a GPS/BDS ionosphere-free combination, ∇Δd_(IF,GC)=Δd_(IF,C)−Δd_(IF,G) represents a pseudorange differential inter-system bias of the GPS/BDS ionosphere-free combination, ∇Δϕ_(IF,G) ¹ ^(G) ^(,s) represents a carrier observation of the GPS intra-system double-difference ionosphere-free combination, ∇Δρ_(G) ¹ ^(G) ^(,s) represents a GPS intra-system double-difference station satellite distance, ΔN_(IF,G) ¹ ^(G) ^(,s) represents an ambiguity of the GPS intra-system double-difference ionosphere-free combination, ∇ΔT_(G) ¹ ^(G) ^(,s) represents a GPS intra-system double-difference troposphere delay, ∇Δε_(IF,G) ¹ ^(G) ^(,s) represents a carrier observation noise of the GPS intra-system double-difference ionosphere-free combination, ∇ΔP_(IF,G) ¹ ^(G) ^(,s) represents a pseudorange observation of the GPS intra-system double-difference ionosphere-free combination, and ∇Δe_(IF,G) ¹ ^(G) ^(,s) represents a carrier observation noise of the GPS intra-system double-difference ionosphere-free combination; and ∇Δϕ_(IF,GC) ¹ ^(G) ^(,q) represents a carrier observation of the GPS/BDS inter-system double-difference ionosphere-free combination, ∇Δρ_(GC) ¹ ^(G) ^(,q) represents a GPS/BDS inter-system double-difference distance between satellites and stations, ∇ΔN_(IF,GC) ¹ ^(G) ^(,q) represents an ambiguity of the GPS/BDS inter-system double-difference ionosphere-free combination, ΔN_(IF,G) ¹ ^(G) represents an ambiguity of the inter-station single-difference ionosphere-free combination of the GPS reference satellite, ∇ΔT_(GC) ¹ ^(G) ^(,q) represents a GPS/BDS inter-system double-difference troposphere delay, ∇Δε_(IF,GC) ¹ ^(G) ^(,q) represents a carrier observation of the GPS/BDS inter-system double-difference ionosphere-free combination, ∇Δ_(IF,GC) ¹ ^(G) ^(,q) represents a pseudorange observation of the GPS/BDS inter-system double-difference ionosphere-free combination, and ∇Δe_(IF,GC) ¹ ^(G) ^(,q) represents a pseudorange observation of the GPS/BDS inter-system double-difference ionosphere-free combination.

In step 13, a BDS reference satellite is selected to construct intra-system double-difference wide-lane ambiguity calculation models of GPS and BDS:

assuming that a BDS satellite 1_(C) is used as a BDS reference satellite, then the respective intra-system double-difference wide-lane ambiguity calculation models of the GPS and the BDS are represented as:

$\begin{matrix} {{\nabla\Delta_{{WL},G}^{1_{G},s}} = {{{\nabla\Delta}\; \phi_{{WL},G}^{1_{G},s}} - \frac{{f_{1,G}{\nabla\Delta}\; P_{1,G}^{1_{G},s}} + {f_{2,G}{\nabla\Delta}\; P_{2,G}^{1_{G},s}}}{\lambda_{{WL},G}\left( {f_{1,G} + f_{2,G}} \right)}}} & (12) \\ {{\nabla\Delta_{{WL},C}^{1_{C},q}} = {{{\nabla\Delta}\; \phi_{{WL},C}^{1_{C},q}} - \frac{{f_{1,C}{\nabla\Delta}\; P_{1,C}^{1_{C},q}} + {f_{2,C}{\nabla\Delta}\; P_{2,C}^{1_{C},q}}}{\lambda_{{WL},C}\left( {f_{1,C} + f_{2,C}} \right)}}} & (13) \end{matrix}$

wherein, equations (12) and (13) are respectively the GPS intra-system double-difference wide-lane ambiguity calculation model and the BDSS intra-system double-difference wide-lane ambiguity calculation model.

∇ΔN_(WL,G) ¹ ^(G) ^(,s) represents a GPS double-difference wide-lane ambiguity, ∇Δϕ_(WL,G) ¹ ^(G) ^(,s) represents a GPS double-difference wide-lane carrier observation (weekly), ∇ΔP_(1,G) ¹ ^(G) ^(,s) represents a GPS L1 double-difference pseudorange observation, ∇ΔP_(2,G) ¹ ^(G) ^(,s) represents a GPS L2 double-difference pseudorange observation, and λ_(WL,G) represents a GPS wide-lane wavelength; and ∇ΔN_(WL,C) ¹ ^(C) ^(,q) represents a BDS double-difference wide-lane ambiguity, ∇Δϕ_(WL,C) ¹ ^(C) ^(,q) represents a BDS double-difference wide-lane carrier observation (weekly), ∇ΔP_(1,C) ¹ ^(C) ^(,q) represents a BDS B1 double-difference pseudorange observation, ∇ΔP_(2,C) ¹ ^(C) ^(,q) represents a BDS B2 double-difference pseudorange observation, and λ_(WL,C) represents a BDS wide-lane wavelength.

Multi-epoch smooth rounding is performed on equations (12) and (13) to obtain a double-difference wide-lane whole-cycle ambiguity, which is represented as:

$\begin{matrix} {{{{\nabla\overset{\Cap}{\Delta}}\; {\overset{\_}{N}}_{{WL},G}^{1_{G},s}} = {{round}\left( {\frac{1}{k}{\sum\limits_{i = 1}^{k}{{\nabla\overset{\Cap}{\Delta}}\; N_{{WL},G}^{1_{G},s}}}} \right)}}{{{\nabla\overset{\Cap}{\Delta}}{\overset{\_}{N}}_{{WL},C}^{1_{C},q}} = {{{{round}\left( {\frac{1}{k}{\sum\limits_{i = 1}^{k}{{\nabla\overset{\Cap}{\Delta}}\; N_{{WL},C}^{1_{C},q}}}} \right)}k} \in N^{+}}}} & (14) \end{matrix}$

wherein, ∇{circumflex over (Δ)}N _(WL,G) ¹ ^(G) ^(,s) and ∇{circumflex over (Δ)}_(WL,C) ¹ ^(C) ^(,q) are respectively the double-difference wide-lane whole-cycle ambiguities of the GPS and the BDS obtained by multi-epoch smooth rounding, round represents a rounding operator, and k represents an epoch number.

The realizing the decorrelation of the inter-system bias parameter in the ionosphere-free combination form with the single-difference and double-difference ambiguities in the step 2 comprises the following steps.

In step 21, the ambiguity of the GPS/BDS inter-system double-difference ionosphere-free combination is reparametrized according to the BDS reference satellite selected in the step 13:

according to the step 12, the ambiguity of the GPS/BDS inter-system double-difference ionosphere-free combination is represented as:

∇ΔN _(IF,G) ¹ ^(G) ^(,q)=(ΔN _(IF,C) ^(q) −ΔN _(IF,C) ¹ ^(C) )+(ΔN _(IF,C) ¹ ^(C) −ΔN _(IF,G) ¹ ^(G) )=∇ΔN _(IF,C) ¹ ^(C) ^(,q) +∇ΔN _(IF,GC) ¹ ^(G) ¹ ^(C)   (15)

wherein, ΔN_(IF,C) ¹ ^(C) represents an ambiguity of an inter-station single-difference ionosphere-free combination of a BDS reference satellite, ∇ΔN_(IF,C) ¹ ^(C) ^(,q) represents an ambiguity of a BDS intra-system double-difference ionosphere-free combination, and ∇ΔN_(IF,C) ¹ ^(G) ¹ ^(C) represents ambiguities of the GPS/BDS inter-system double-difference ionosphere-free combinations of the BDS reference satellite and the GPS reference satellite.

According to equation (15), equation (10) is represented as:

∇Δϕ_(IF,GC) ¹ ^(G) ^(,q)=∇Δρ_(GC) ¹ ^(G) ^(,q)+λ_(NL,C) ∇ΔN _(IF,C) ¹ ^(C) ^(,q)+λ_(NL,C) ∇ΔN _(IF,GC) ¹ ^(G) ¹ ^(C) +(λ_(IF,C)−λ_(IF,G))ΔN _(IF,G) ¹ ^(G) +λ_(NL,C)∇Δδ_(IF,GC) +∇ΔT _(GC) ¹ ^(G) ^(,q)+∇Δε_(IF,GC) ¹ ^(G) ^(,q)  (16)

wherein, in equation (10), ∇ΔN_(IF,GC) ¹ ^(G) ¹ ^(C) , ΔN_(IF,G) ¹ ^(G) and ∇Δδ_(IF,GC) are parameters shared by all BDS satellites and are linearly correlated.

In step 22, the shared parameters are combined and the ionosphere-free combination carrier difference inter-system bias is reparametrized to realize parameter decorrelation:

according to equation (16), an observation equation of the GPS/BDS inter-system double-difference ionosphere-free combination after the shared parameters are combined is represented as:

∇Δϕ_(IFGC) ¹ ^(G) ^(,q)=∇Δρ_(GC) ¹ ^(G) ^(,q)+λ_(NL,C) ∇ΔN _(NL,C) ¹ ^(C) ^(,q)+λ_(NL,C)∇Δδ _(IF,GC) +∇ΔT _(GC) ¹ ^(G) ^(,q)+∇Δε_(IF,GC) ¹ ^(G) ^(,q)  (17)

wherein,

${{{\nabla\Delta}\; {\overset{\_}{\delta}}_{{IF},{GC}}} = {{{\nabla\Delta}\; N_{{IF},{GC}}^{1_{G}1_{C}}} + {{\nabla\Delta}\; \delta_{{IF},{GC}}} + {\left( {1 - \frac{\lambda_{{NL},C}}{\lambda_{{NL},G}}} \right)\Delta \; N_{{IF},G}^{1_{G}}}}},$

∇Δδ _(IF,GC) represents an ionosphere-free combination carrier difference inter-system bias after reparameterization, and a ∇Δδ _(IF,GC) form will be used as the ionosphere-free combination carrier difference inter-system bias hereinafter.

The performing the reference conversion to realize the continuous estimability of the ionosphere-free combination difference inter-system bias in the step 3 comprises the following steps.

In step 31, GPS reference conversion is performed:

assuming that the GPS reference satellite is converted from 1_(G) into i_(G) at a t^(th) epoch, a corresponding ionosphere-free combination carrier difference inter-system bias ∇Δδ _(IF,GC) (t) of the t^(th) epoch is:

$\begin{matrix} {{{\nabla\Delta}\; {{\overset{\_}{\delta}}_{{IF},{GC}}(t)}} = {{{{\nabla\Delta}\; {{\overset{\_}{\delta}}_{{IF},{GC}}\left( {t - 1} \right)}} - {\frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}\Delta \nabla_{{IF},G}^{1_{G},i_{G}}}} = {{{\nabla\Delta}\; \delta_{{IF},{GC}}} + {{\nabla\Delta}\; N_{{IF},{GC}}^{i_{G},1_{C}}} + {\left( {1 - \frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}} \right)\Delta \; N_{{IF},G}^{i_{G}}}}}} & (18) \end{matrix}$

wherein, ∇Δδ _(IF,GC) (t−1) is an ionosphere-free combination carrier difference inter-system bias of a (t−1)^(th) epoch.

In step 32, BDS reference conversion is performed:

assuming that the BDS reference satellite is converted from 1_(C) into i_(C) at a j^(th) epoch, while the GPS reference satellite at the moment is i_(G) in the step 41, then a corresponding ionosphere-free combination carrier difference inter-system bias ∇Δδ _(IF,GC)(j) of the j^(th) epoch is:

$\begin{matrix} {{{\nabla\Delta}\; {{\overset{\_}{\delta}}_{{IF},{GC}}(j)}} = {{{{\nabla\Delta}\; {{\overset{\_}{\delta}}_{{IF},{GC}}\left( {j - 1} \right)}} + {{\nabla\Delta}\; N_{{IF},{GC}}^{1_{C},j_{C}}}} = {{{\nabla\Delta}\; \delta_{{IF},{GC}}} + {{\nabla\Delta}\; N_{{IF},{GC}}^{i_{G},i_{C}}} + {\left( {1 - \frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}} \right)\Delta \; N_{{IF},G}^{i_{G}}}}}} & (19) \end{matrix}$

wherein, ∇Δδ _(IF,GC) (j−1) is an ionosphere-free combination difference carrier inter-system bias of a (j−1)^(th) epoch. So far, the continuous estimability of the ionosphere-free combination carrier difference inter-system bias is realized.

The separating the base carrier ambiguity by using the ionosphere-free combination and the wide-lane ambiguity in the step 4 comprises the following steps.

In step 41, the ambiguity of the GPS L1 is separated from the ambiguity of the BDS B1 according to the wide-lane ambiguity obtained in the step 13 by combining the ionosphere-free combination with a wide-lane combination:

$\begin{matrix} {{\nabla\Delta_{1,G}^{1_{G},s}} = {{\nabla\Delta_{{IF},G}^{1_{G},s}} - {\frac{f_{2,G}}{f_{1,G} - f_{2,G}}{\nabla\Delta}\; {\overset{\_}{N}}_{{WL},G}^{1_{G},s}}}} & (20) \\ {{\nabla\Delta_{1,C}^{1_{C},q}} = {{\nabla\Delta_{{IF},C}^{1_{C},q}} - {\frac{f_{2,C}}{f_{1,C} - f_{2,C}}{\nabla\Delta}\; {\overset{\_}{N}}_{{WL},C}^{1_{C},s}}}} & (21) \end{matrix}$

wherein, ∇ΔN_(1,G) ¹ ^(G) ^(,s) is a separated ambiguity float solution of the GPS L1, and ∇ΔN_(1,C) ¹ ^(C) ^(,q) is a separated ambiguity float solution of the BDS B1. Integer-ambiguity solutions ∇ΔN _(1,G) ¹ ^(G) ^(,s) and ∇ΔN_(1,C) ¹ ^(C) ^(,q) of the GPS L1 and the BDS B1 are searched by a least-squares ambiguity decorrelation adjustment (LAMBDA) method.

In step 42, according to the wide-lane ambiguity obtained in the step 13 and the ambiguity of the GPS L1 and the ambiguity of the BDS B1 obtained in the step 41, an integer-ambiguity solution of the GPS L2 and an integer-ambiguity solution of the BDS L2 are calculated:

∇Δ N _(2,G) ¹ ^(G) ^(,s) =∇ΔN _(1,G) ¹ ^(G) ^(,s) −∇ΔN _(WL,G) ¹ ^(G) ^(,s) ,∇ΔN _(2,C) ¹ ^(C) ^(,q) =∇ΔN _(1,C) ¹ ^(C) ^(,q) −∇ΔN _(WL,C) ¹ ^(C) ^(,q)  (22)

wherein, ∇ΔN_(2,G) ¹ ^(G) ^(,s) and ∇ΔN _(2,C) ¹ ^(C) ^(,q) are respectively the integer-ambiguity solutions of the GPS L2 and the BDS B2.

The forming the ionosphere-free combination by using the base carrier observations to perform high-precision positioning in the step 5 comprises the following step.

In step 51, the double-difference ionosphere-free combination is formed according to the step 21 based on the integer-ambiguity solutions and the carrier observations obtained in the step 41 and the step 42, and the formed ionosphere-free combination and the ionosphere-free combination carrier difference inter-system bias obtained in the step 2 are substituted into the equations (5) and (7) for positioning. It shall be noted that the ionosphere-free combination carrier difference inter-system bias must be consistent with the reference satellites of the equations (5) and (7).

The positioning bias is shown in FIGS. 1 to 6, wherein FIGS. 1, 3 and 5 respectively illustrate positioning bias diagrams of a loose combination in N/E/U directions, and FIGS. 2, 4 and 5 respectively illustrate positioning bias diagrams of a tight combination in the N/E/U directions.

According to method, the GPS is used as the reference system to form the ionosphere-free combination to perform GPS/BDS inter-system tight combined carrier differential positioning. The inter-system bias in the carrier ionosphere-free combination form is estimated in real time, the base carrier ambiguity is separated by using the ionosphere-free combination and the wide-lane combination, and the ionosphere-free combination is finally formed by using the base carrier to perform tight combined differential positioning.

The foregoing is only the specific embodiments in the present invention, but the protection scope of the present invention is not limited to the embodiments. Any changes or substitutions that can be understood and thought of by those skilled in the art within the technical scope disclosed by the present invention shall be included within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims. 

1. A tightly combined GPS/BDS carrier differential positioning method, wherein the method comprises the following steps: step 1: selecting a GPS reference satellite to construct a GPS intra-system double-difference ionosphere-free combination model, a GPS/BDS inter-system double-difference ionosphere-free combination model and a GPS/BDS intra-system double-difference wide-lane ambiguity calculation model; step 2: realizing decorrelation of an inter-system bias parameter with single-difference and double-difference ambiguities in ionosphere-free combinations; step 3: performing reference conversion to realize a continuous estimability of an ionosphere-free combination difference inter-system bias; step 4: separating a base carrier ambiguity by using an ionosphere-free combination and a fixed wide-lane ambiguity; and step 5: forming the ionosphere-free combination by using base carrier observations to perform high-precision positioning.
 2. The tightly combined GPS/BDS carrier differential positioning method according to claim 1, wherein the step 1 further comprises: step 11: constructing an inter-station single-difference ionosphere-free combination model: $\begin{matrix} {{\Delta \; \varphi_{{IF},G}^{s}} = {{\Delta \; \rho_{G}^{s}} + {\Delta \; {dt}} + {\lambda_{{NL},G}\left( {{\Delta\delta}_{{IF},G} + {\Delta \; N_{{IF},G}^{s}}} \right)} + {\Delta \; T_{G}^{s}} + {\Delta \; ɛ_{{IF},G}^{s}}}} & (1) \\ {{\Delta \; P_{{IF},G}^{s}} = {{\Delta \; \rho_{G}^{s}} + {\Delta \; {dt}} + {\Delta \; d_{{IF},G}} + {\Delta \; T_{G}^{s}} + {\Delta \; e_{{IF},G}^{s}}}} & (2) \\ {{\Delta \; \varphi_{{IF},C}^{q}} = {{\Delta \; \rho_{C}^{q}} + {\Delta \; {dt}} + {\lambda_{{NL},C}\left( {{\Delta \; \delta_{{IF},C}} + {\Delta \; N_{{IF},C}^{q}}} \right)} + {\Delta \; T_{C}^{q}} + {\Delta \; ɛ_{{IF},C}^{q}}}} & (3) \\ {{\Delta \; P_{{IF},C}^{q}} = {{\Delta \; \rho_{C}^{q}} + {\Delta \; {dt}} + {\Delta \; d_{{IF},C}} + {\Delta \; T_{C}^{q}} + {\Delta \; e_{{IF},C}^{q}}}} & (4) \\ {{{\Delta \; \varphi_{{IF},G}^{s}} = {\frac{f_{1,G}^{2}\Delta \; \varphi_{1,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}} - \frac{f_{2,G}^{2}\Delta \; \varphi_{2,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}}}}{{\Delta \; N_{{IF},G}^{s}} = {\frac{f_{1,G}^{2}\Delta \; N_{1,G}^{s}}{{f_{1,G}^{2} - f_{2,G}^{2}}\;} - \frac{f_{2,G}^{2}\Delta \; N_{2,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}}}}} & (5) \\ {{{\Delta \; \varphi_{{IF},C}^{q}} = {\frac{f_{1,C}^{2}\Delta \; \varphi_{1,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}} - \frac{f_{2,G}^{2}\Delta \; \varphi_{2,C}^{q}}{f_{1,G}^{2} - f_{2,C}^{2}}}}{{\Delta \; N_{{IF},C}^{q}} = {\frac{f_{1,C}^{2}\Delta \; N_{1,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}} - \frac{f_{2,C}^{2}\Delta \; N_{2,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}}}}} & (6) \\ {{{\Delta \; P_{{IF},G}^{s}} = {\frac{f_{1,G}^{2}\Delta \; P_{1,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}} - \frac{f_{2,G}^{2}\Delta \; P_{2,G}^{s}}{f_{1,G}^{2} - f_{2,G}^{2}}}}{{\Delta \; P_{{IF},C}^{q}} = {\frac{f_{1,C}^{2}\Delta \; P_{1,C}^{q}}{f_{1,C}^{2} - f_{2,C}^{2}} - \frac{f_{2,G}^{2}\Delta \; P_{2,C}^{q}}{f_{1,G}^{2} - f_{2,C}^{2}}}}} & (7) \end{matrix}$ wherein equations (1) and (2) respectively represent a carrier observation equation and a pseudorange observation equation of a GPS inter-station single-difference ionosphere-free combination, equations (3) and (4) respectively represent a subcarrier observation equation and a pseudorange observation equation of a BDS inter-station single-difference ionosphere-free combination, equation (5) represents a GPS inter-station single-difference ionosphere-free carrier observation and a GPS inter-station single-difference ionosphere-free ambiguity, equation (6) represents a BDS inter-station single-difference ionosphere-free carrier observation value and a BDS inter-station single-difference ionosphere-free ambiguity, and equation (7) represents a GPS inter-station single-difference pseudorange ionosphere-free combination and a BDS inter-station single-difference pseudorange ionosphere-free combination; wherein s=1_(G), 2_(G), . . . , m_(G), m_(G) represents a number of GPS satellites, Δϕ_(IF,G) ^(s) represents a carrier observation of an inter-station single-difference ionosphere-free combination of the GPS satellites Δρ_(G) ^(s) represents a single-difference distance between a station and a GPS satellite, Δdt represents an inter-station single-difference receiver clock bias, λ_(NL,G) represents a GPS narrow-lane wavelength, Δδ_(IF,G) represents a carrier hardware delay of an inter-station single-difference ionosphere-free combination of a GPS satellite receiver, ΔN_(IF,G) ^(s) represents an ambiguity of the inter-station single-difference ionosphere-free combination of the GPS satellites, ΔT_(G) ^(s) represents an inter-station single-difference troposphere delay of a GPS satellite, Δε_(IF,G) ^(s) represents a measurement noise of the inter-station single-difference ionosphere-free combination of the GPS satellite, ΔP_(IF,G) ^(s) represents a pseudorange observation of the inter-station single-difference ionosphere-free combination of the GPS satellites, Δd_(IF,G) represents a pseudorange hardware delay of the inter-station single-difference ionosphere-free combination of the GPS satellite receiver end, and Δe_(IF,G) ^(s) represents a pseudorange measured noise of the inter-station single-difference ionosphere-free combination of the GPS satellites; q=1_(C), 2_(C), n_(C), n_(C) represents a number of BDS satellites, Δϕ_(IF,C) ^(q) represents a carrier observation value of an inter-station single-difference ionosphere-free combination of a BDS satellite q, Δρ_(C) ^(q) represents an inter-station single-difference station satellite distance of the BDS satellite q, λ_(NL,C) represents a BDS narrow-lane wavelength, Δδ_(IF,C) represents a carrier hardware delay of an inter-station single-difference ionosphere-free combination of the BDS receiver, ΔN_(IF,C) ^(q) represents an ambiguity of the inter-station single-difference ionosphere-free combination of the BDS satellite q, ΔT_(C) ^(q) represents an inter-station single-difference troposphere delay of the BDS satellite q, Δε_(IF,C) ^(q) represents a measurement noise of the inter-station single-difference ionosphere-free combination of the BDS satellite q, ΔP_(IF,C) ^(q) represents a pseudorange observation of the inter-station single-difference ionosphere-free combination of the BDS satellite q, Δd_(IF,C) represents a pseudorange hardware delay of the inter-station single-difference ionosphere-free combination of a BDS satellite receiver, and Δe_(IF,C) ^(q) represents a pseudorange measurement noise of the inter-station single-difference ionosphere-free combination of the BDS satellite q; Δϕ_(1,G) ^(s) represents an inter-station single-difference carrier observation on a L1 frequency of a GPS satellite s, Δϕ_(2,G) ^(s) represents an inter-station single-difference carrier observation on a L2 frequency of the GPS satellite s, ΔN_(1,G) ^(s) represents an inter-station single-difference ambiguity on the L1 frequency of the GPS satellite s, ΔN_(2,G) ^(s) represents an inter-station single-difference ambiguity on the L2 frequency of the GPS satellite s, ΔP_(1,G) ^(s) represents an inter-station single-difference pseudorange observation on the L1 frequency of the GPS satellites s, ΔP_(2,G) ^(s) represents an inter-station single-difference pseudorange observation on the L2 frequency of the GPS satellites s, f_(1,G) represents a GPS L1 frequency, and f_(2,G) represents a GPS L2 frequency; and Δϕ_(1,C) ^(q) represents an inter-station single-difference carrier observation on a B1 frequency of the BDS satellite q, Δϕ_(2,C) ^(q) represents an inter-station single-difference carrier observation on a B2 frequency of the BDS satellite q, ΔN_(1,C) ^(q) represents the inter-station single-difference ambiguity on the B1 frequency of the BDS satellite q, ΔN_(1,C) ^(q) represents the inter-station single-difference ambiguity on the B2 frequency of the BDS satellite q, ΔP_(1,C) ^(q) represents the inter-station single-difference pseudorange observation on the B1 frequency of the BDS satellite q, ΔP_(2,C) ^(q) represents the inter-station single-difference pseudorange observation on the B2 frequency of the BDS satellite q, f_(1,C) represents a B1 frequency of BDS, and f_(2,C) represents a B2 frequency of BDS; step 12: selecting the GPS reference satellite to construct the GPS intra-system double-difference ionosphere-free combination model and the GPS/BDS inter-system double-difference ionosphere-free combination model according to the inter-station single-difference ionosphere-free combination model constructed in the step 11: wherein when a GPS satellite 1_(G) is used as a reference satellite, equations (8) and (9) representing GPS intra-system double-difference ionosphere-free combination models, and equations (10) and (11) representing GPS/BDS inter-system double-difference ionosphere-free combination models: $\begin{matrix} {\mspace{20mu} {{\nabla{\Delta\varphi}_{{IF},G}^{1_{G},s}} = {{{\nabla\Delta}\; \rho_{G}^{1_{G},s}} + {\lambda_{{NL},G}\Delta \; N_{{IF},G}^{1_{G},s}} + {\Delta {\nabla T_{G}^{1_{G},s}}} + {{\nabla\Delta}\; ɛ_{{IF},G}^{1_{G},s}}}}} & (8) \\ {\mspace{20mu} {{{\nabla\Delta}\; P_{{IF},G}^{1_{G},s}} = {{{\nabla\Delta}\; \rho_{G}^{1_{G},s}} + \; {{\nabla\Delta}\; T_{G}^{1_{G},s}} + {{\nabla\Delta}\; e_{{IF},G}^{1_{G},s}}}}} & (9) \\ \begin{matrix} {{{\nabla\Delta}\; \varphi_{{IF},{GC}}^{1_{G},s}} = {{\Delta \; \varphi_{{IF},C}^{q}} - {\Delta \; \varphi_{{IF},G}^{1_{G}}}}} \\ {= {{\nabla{\Delta\rho}_{GC}^{1_{G},q}} + {\lambda_{{NL},C}{\nabla\Delta}\; N_{{IF},{GC}}^{1_{G},q}} + {\left( {\lambda_{{NL},C} - \lambda_{{NL},G}} \right)\Delta \; N_{{IF},G}^{1_{G\;}}} +}} \\ {{{\lambda_{{NL},C}{\nabla\Delta}\; \delta_{{IF},{GC}}} + {{\nabla\Delta}\; T_{GC}^{1_{G},q}} + {{\nabla\Delta}\; ɛ_{{IF},{GC}}^{1_{G},q}}}} \end{matrix} & (10) \\ {{{\nabla\Delta}\; P_{{IF},{GC}}^{1_{G},q}} = {{{\Delta \; P_{{IF},C}^{q}} - {\Delta \; P_{{IF},G}^{1_{G}}}} = {{{\nabla\Delta}\; \rho_{GC}^{1_{G},q}} + {{\nabla\Delta}\; d_{{IF},{GC}}} + {{\nabla\Delta}\; T_{GC}^{1_{G},q}} + {{\nabla\Delta}\; e_{{IF},{GC}}^{1_{G},q}}}}} & (11) \end{matrix}$ wherein, ∇Δϕ_(IF,G) ¹ ^(G) ^(,s) represents a carrier observation of the GPS intra-system double-difference ionosphere-free combination, ∇Δρ_(G) ¹ ^(G) ^(,s) represents a GPS intra-system double-difference distance between stations and satellites, ΔN_(IF,G) ¹ ^(G) ^(,s) represents a double-difference ambiguity of a GPS intra-system ionosphere-free combination, ∇ΔT_(G) ¹ ^(G) ^(,s) represents a GPS intra-system double-difference troposphere delay, ∇Δε_(IF,G) ¹ ^(G) ^(,s) represents a carrier observation of the GPS intra-system double-difference ionosphere-free combination, ∇ΔP_(IF,G) ¹ ^(G) ^(,s) represents a pseudorange observation of the GPS intra-system double-difference ionosphere-free combination, and ∇Δe_(IF,G) ¹ ^(G) ^(,s) represents a carrier measurement noise of the GPS intra-system double-difference ionosphere-free combination; and ∇Δϕ_(IF,GC) ¹ ^(G) ^(,q) represents a carrier observation of the GPS/BDS inter-system double-difference ionosphere-free combination, ∇Δρ_(GC) ¹ ^(G) ^(,q) represents a GPS/BDS inter-system double-difference distance between satellites and stations, ∇ΔN_(IF,GC) ¹ ^(G) ^(,q) represents an ambiguity of the GPS/BDS inter-system double-difference ionosphere-free combination, ΔN_(IF,G) ¹ ^(G) represents an ambiguity of the inter-station single-difference ionosphere-free combination of the GPS reference satellite, ${\nabla{\Delta\delta}_{{IF},{GC}}} = {{\Delta \; \delta_{{IF},C}} - {\frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}\Delta \; \delta_{{IF},G}}}$ represents a carrier difference inter-system bias of the GPS/BDS ionosphere-free combination, ∇ΔT_(GC) ¹ ^(G) ^(,q) represents a GPS/BDS inter-system double-difference troposphere delay, ∇Δε_(IF,GC) ¹ ^(G) ^(,q) represents a carrier observation of the GPS/BDS inter-system double-difference ionosphere-free combination, ∇ΔP_(IF,GC) ¹ ^(G) ^(,q) represents a pseudorange observation of the GPS/BDS inter-system double-difference ionosphere-free combination, ∇Δd_(IF,GC)=Δd_(IF,C)−Δd_(IF,G) represents a GPS/BDS pseudorange differential inter-system bias of the ionosphere-free combination, and ∇Δe_(IF,GC) ¹ ^(G) ^(,q) represents a pseudorange observation of the GPS/BDS inter-system double-difference ionosphere-free combination; and step 13: selecting a BDS reference satellite to construct intra-system double-difference wide-lane ambiguity calculation models: wherein when a BDS satellite 1_(C) is used as the BDS reference satellite, the respective intra-system double-difference wide-lane ambiguity calculation models of the GPS and the BDS being: $\begin{matrix} {{{\nabla\Delta}\; N_{{WL},G}^{1_{G},s}} = {{\nabla{\Delta\phi}_{{WL},G}^{1_{G},s}} - \frac{{f_{1,G}{\nabla\Delta}\; P_{1,G}^{1_{G},s}} + {f_{2,G}{\nabla\Delta}\; P_{2,G}^{1_{G},s}}}{\lambda_{{WL},G}\left( {f_{1,G} + f_{2,G}} \right)}}} & (12) \\ {{{\nabla\Delta}\; N_{{WL},C}^{1_{C},q}} = {{\nabla{\Delta\phi}_{{WL},C}^{1_{C},q}} - \frac{{f_{1,C}{\nabla\Delta}\; P_{1,C}^{1_{C},q}} + {f_{2,C}{\nabla\Delta}\; P_{2,C}^{1_{C},q}}}{\lambda_{{WL},C}\left( {f_{1,C} + f_{2,C}} \right)}}} & (13) \end{matrix}$ wherein, ∇ΔN_(WL,G) ¹ ^(G) ^(,s) represents a GPS double-difference wide-lane ambiguity, ∇Δϕ_(WL,G) ¹ ^(G) ^(,s) represents a GPS double-difference wide-lane carrier observation, ∇ΔP_(1,G) ¹ ^(G) ^(,s) represents a GPS L1 double-difference pseudorange observation, ∇ΔP_(2,G) ¹ ^(G) ^(,s) represents a GPS L2 double-difference pseudorange observation, and λ_(WL,G) represents a GPS wide-lane wavelength; and ∇ΔN_(WL,C) ¹ ^(C) ^(,q) represents a BDS double-difference wide-lane ambiguity, ∇Δφ_(WL,C) ¹ ^(C) ^(,q) represents a BDS double-difference wide-lane carrier observation, ∇ΔP_(1,C) ¹ ^(C) ^(,q) represents a BDS B1 double-difference pseudorange observation, ∇ΔP_(2,C) ¹ ^(C) ^(,q) represents a BDS B2 double-difference pseudorange observation, and λ_(WL,C) represents a BDS wide-lane wavelength; and performing multi-epoch smooth rounding on equations (12) and (13) to obtain double-difference wide-lane whole-cycle ambiguities: $\begin{matrix} {{{{\nabla\overset{\Cap}{\Delta}}\; {\overset{\_}{N}}_{{WL},G}^{1_{G},s}} = {{round}\left( {\frac{1}{k}{\sum\limits_{i = 1}^{k}{{\nabla\overset{\Cap}{\Delta}}N_{{WL},G}^{1_{G},s}}}} \right)}}{{{\nabla\overset{\Cap}{\Delta}}\; {\overset{\_}{N}}_{{WL},C}^{1_{C},q}} = {{round}\left( {\frac{1}{k}{\sum\limits_{i = 1}^{k}{{\nabla\overset{\Cap}{\Delta}}N_{{WL},C}^{1_{C},q}}}} \right)}}{k \in N^{+}}} & (14) \end{matrix}$ wherein, ∇{circumflex over (Δ)}N _(WL,G) ¹ ^(G) ^(,s) and ∇{circumflex over (Δ)}N _(WL,C) ¹ ^(C) ^(,q) are respectively the double-difference wide-lane whole-cycle ambiguities of the GPS and the BDS obtained by multi-epoch smooth rounding, round represents a rounding operator, and k represents an epoch number.
 3. The tightly combined GPS/BDS carrier differential positioning method according to claim 2, wherein the step 2 further comprises: step 21: reparameterizing the ambiguity of the GPS/BDS inter-system double-difference ionosphere-free combination according to the BDS reference satellite selected in the step 13: according to the step 12, the ambiguity of the GPS/BDS inter-system double-difference ionosphere-free combination being represented as: ∇ΔN _(IF,G) ¹ ^(G) ^(,q)=(ΔN _(IF,C) ^(q) −ΔN _(IF,C) ¹ ^(C) )+(ΔN _(IF,C) ¹ ^(C) −ΔN _(IF,G) ¹ ^(G) )=∇ΔN _(IF,C) ¹ ^(C) ^(,q) +∇ΔN _(IF,GC) ¹ ^(G) ¹ ^(C)   (15) wherein, ΔN_(IF,C) ¹ ^(C) represents an ambiguity of an inter-station single-difference ionosphere-free combination of a BDS reference satellite, ∇ΔN_(IF,C) ¹ ^(C) ^(,q) represents an ambiguity of a BDS intra-system double-difference ionosphere-free combination, and ∇ΔN_(IF,GC) ¹ ^(G) ¹ ^(C) represents ambiguities of the GPS/BDS inter-system double-difference ionosphere-free combinations of the BDS reference satellite and the GPS reference satellite; according to equation (15), equation (10) being represented as: ∇Δϕ_(IF,GC) ¹ ^(G) ^(,q)=∇Δρ_(GC) ¹ ^(G) ^(,q)+λ_(NL,C) ∇ΔN _(IF,C) ¹ ^(C) ^(,q)+λ_(NL,C) ∇ΔN _(IF,GC) ¹ ^(G) ¹ ^(C) +(λ_(IF,C)−λ_(IF,G))ΔN _(IF,G) ¹ ^(G) +λ_(NL,C)∇Δδ_(IF,GC) +∇ΔT _(GC) ¹ ^(G) ^(,q)+∇Δε_(IF,GC) ¹ ^(G) ^(,q)  (16) wherein, in equation (10), ∇ΔN_(IF,GC) ¹ ^(G) ¹ ^(C) , ΔN_(IF,G) ¹ ^(G) and ∇Δδ_(IF,GC) are parameters shared by all BDS satellites and are linearly correlated; and step 22: combining the shared parameters and reparametrizing an ionosphere-free combination carrier difference inter-system bias to realize parameter decorrelation: according to equation (16), an observation equation of the GPS/BDS inter-system double-difference ionosphere-free combination after the shared parameters are combined being represented as: ∇Δϕ_(IFGC) ¹ ^(G) ^(,q)=∇Δρ_(GC) ¹ ^(G) ^(,q)+λ_(NL,C) ∇ΔN _(NL,C) ¹ ^(C) ^(,q)+λ_(NL,C)∇Δδ _(IF,GC) +∇ΔT _(GC) ¹ ^(G) ^(,q)+∇Δε_(IF,GC) ¹ ^(G) ^(,q)  (17) wherein, ∇Δδ _(IF,GC) represents the ionosphere-free combination carrier difference inter-system bias after reparameterization, and ${{\nabla\Delta}{\overset{\_}{\delta}}_{{IF},{GC}}} = {{{\nabla\Delta}\; N_{{IF},{GC}}^{1_{G}1_{C}}} + {\nabla{\Delta\delta}_{{IF},{GC}}} + {\left( {1 - \frac{\lambda_{{NL},C}}{\lambda_{{NL},G}}} \right)\Delta \; {N_{{IF},G}^{1_{G}}.}}}$
 4. The tightly combined GPS/BDS carrier differential positioning method according to claim 3, wherein the step 3 further comprises: step 31: performing GPS reference conversion: assuming that the GPS reference satellite is converted from 1_(G) into i_(G) at a t^(th) epoch, a corresponding ionosphere-free combination carrier difference inter-system bias ∇Δδ _(IF,GC) (t) of the t^(th) epoch being: $\begin{matrix} {{{\nabla\Delta}{{\overset{\_}{\delta}}_{{IF},{GC}}(t)}} = {{{{\nabla\Delta}{{\overset{\_}{\delta}}_{{IF},{GC}}\left( {t - 1} \right)}} - {\frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}\Delta \; {\nabla N_{{IF},G}^{1_{G},i_{G}}}}} = {{{\nabla\Delta}\; \delta_{{IF},{GC}}} + {{\nabla\Delta}\; N_{{IF},{GC}}^{i_{G},1_{C}}} + {\left( {1 - \frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}} \right)\Delta \; N_{{IF},G}^{i_{G}}}}}} & (18) \end{matrix}$ wherein, ∇Δδ _(IF,GC)(t−1) is the ionosphere-free combination carrier difference inter-system bias of the (t−1)^(th) epoch; and step 32: performing BDS reference conversion: assuming that the BDS reference satellite is converted from 1_(C) into i_(C) at a j^(th) epoch, while the GPS reference satellite at the moment being i_(G) in the step 31, then a corresponding ionosphere-free combination carrier difference inter-system bias ∇Δδ _(IF,GC) (J) of the j^(th) epoch being: $\begin{matrix} {{{\nabla\Delta}{{\overset{\_}{\delta}}_{{IF},{GC}}(j)}} = {{{{\nabla\Delta}{{\overset{\_}{\delta}}_{{IF},{GC}}\left( {j - 1} \right)}} + {{\nabla\Delta}\; N_{{IF},{GC}}^{1_{C},i_{C}}}} = {{\nabla{\Delta\delta}_{{IF},{GC}}} + {{\nabla\Delta}\; N_{{IF},{GC}}^{i_{G},i_{C}}} + {\left( {1 - \frac{\lambda_{{NL},G}}{\lambda_{{NL},C}}} \right)\Delta \; N_{{IF},G}^{i_{G}}}}}} & (19) \end{matrix}$ wherein, ∇Δδ _(IF,GC)(j−1)^(th) is an ionosphere-free combination difference carrier inter-system bias of the (j−1)^(th) epoch; so far, the continuous estimability of the ionosphere-free combination carrier difference inter-system bias is realized.
 5. The tightly combined GPS/BDS carrier differential positioning method according to claim 4, wherein the step 4 further comprises: step 41: separating the ambiguity of the GPS L1 and the ambiguity of BDS B1 according to the wide-lane ambiguity obtained in the step 13 by combining the ionosphere-free combination with a wide-lane combination: $\begin{matrix} {{{\nabla\Delta}\; N_{1,G}^{1_{G},s}} = {{{\nabla\Delta}\; N_{{IF},G}^{1_{G},s}} - {\frac{f_{2,G}}{f_{1,G} - f_{2,G}}{\nabla\Delta}\; {\overset{\_}{N}}_{{WL},G}^{1_{G},s}}}} & (20) \\ {{{\nabla\Delta}\; N_{1,C}^{1_{C},q}} = {{{\nabla\Delta}\; N_{{IF},C}^{1_{C},q}} - {\frac{f_{2,C}}{f_{1,C} - f_{2,C}}{\nabla\Delta}\; {\overset{\_}{N}}_{{WL},C}^{1_{C},q}}}} & (21) \end{matrix}$ wherein, ∇ΔN_(1,G) ¹ ^(G) ^(,s) is a separated ambiguity float solution of the GPS L1, ∇ΔN_(1,C) ¹ ^(C) ^(,q) is a separated ambiguity float solution of the BDS B1, ∇ΔN _(1,G) ¹ ^(G) ^(,s) is an integer-ambiguity solution of the GPS L1, and ∇ΔN _(1,C) ¹ ^(C) ^(,q) is an integer-ambiguity solution of the BDS B1; and step 42: according to the wide-lane ambiguity obtained in the step 13 and the ambiguity of the GPS L1 and the ambiguity of the BDS B1 obtained in the step 41, calculating an integer-ambiguity solution of the GPS L2 and an integer-ambiguity solution of the BDS L2: ∇Δ N _(2,G) ¹ ^(G) ^(,s) =∇ΔN _(1,G) ¹ ^(G) ^(,s) −∇ΔN _(WL,G) ¹ ^(G) ^(,s) ,∇ΔN _(2,C) ¹ ^(C) ^(,q) =∇ΔN _(1,C) ¹ ^(C) ^(,q) −∇ΔN _(WL,C) ¹ ^(C) ^(,q)  (22) wherein, ∇ΔN _(2,G) ¹ ^(G) ^(,s) and ∇ΔN _(2,C) ¹ ^(C) ^(,q) are respectively the integer-ambiguity solutions of the GPS L2 and the BDS B2.
 6. The tightly combined GPS/BDS carrier differential positioning method according to claim 5, wherein the integer-ambiguity solutions ∇ΔN _(1,G) ¹ ^(G) ^(,s) and ∇ΔN _(1,C) ¹ ^(C) ^(,q) of the GPS L1 and the BDS B1 are searched by a least-squares ambiguity decorrelation adjustment (LAMBDA) method.
 7. The tightly combined GPS/BDS carrier differential positioning method according to claim 5, wherein the step 5 further comprises: forming the double-difference ionosphere-free combination according to the step 21 based on the integer-ambiguity solutions and the carrier observations obtained in the step 41 and the step 42, and substituting the formed ionosphere-free combination and the ionosphere-free combination carrier difference inter-system bias obtained in the step 2 into the equations (5) and (7) for positioning. 